There is known, particularly from EP Patent No 0848480, a single-phase motor whose stator is directly formed in a base plate or a main plate made of magnetic material (FIG. 4 in particular), with a hole for the permanent magnet of the rotor being provided in the base plate or main plate. The two poles of the stator are formed by machining an aperture in the base plate or main plate, the machining operation being arranged to form two continuous narrow bridge structures or isthmuses at the periphery of the rotor hole. This arrangement is advantageous for allowing the motor to be positioned anywhere within the base plate or main plate, and for allowing several motors to be arranged in the same base plate or main plate. Further, preferably, elements of the timepiece movement which includes the base plate or main plate are mounted thereon. For example, the bearings of a gear train coupled to the motor are arranged in the base plate or main plate.
However, the aforementioned timepiece movement raises a significant problem. It is difficult to machine the isthmuses separating the two stator poles at the periphery of the rotor hole with the conventional machining techniques used for plates or bridges of a timepiece movement. The isthmuses must be thin to ensure magnetic saturation between the two stator poles. Thus, low tolerance machining must be used to create isthmuses of very small width (in the central part thereof). In a conventional timepiece motor, these isthmuses are obtained by stamping. Stamping isthmuses in a main plate is not without problems and increases the production costs of the plates. Further, the plate has portions, formed by the inner poles located inside apertures, which are fragile and which can easily be bent or broken. A plate generally has to undergo several machining operations and treatments; which involve various manipulations, during which the inner poles risk being damaged. It will be noted that any problem in machining the isthmuses or any inner pole damaged during a manipulation will result in the plate becoming unusable. Plates are expensive. Thus, even for an average industrial yield, the financial loss is high.